High-temperature sensors are used, for example, to measure the temperature in exhaust pipes of gasoline engines or in furnaces. They may be suited to measure temperatures of greater than 500° C. Especially when used in the automobile field, e.g. in exhaust gas cleaning systems, high-temperature sensors of this kind are exposed to high thermal and mechanical (due to the vibrations of the engine) loads. The sensor element for measuring the temperature is, therefore, typically protected by a protective envelope, in particular a protective tube, e.g. of metal.
In particular, high-temperature sensors of this kind may be designed as sheath thermocouples.
DE 10 2008 060 033 A1 discloses a temperature sensor having a thermocouple, which includes a sheathed fireproof cable including a sensor element attached to the cable end facing the sample medium and featuring electric connecting leads that run through a casing tube of the sheathed cable for connecting the sensor element to an electronic evaluation unit. It is proposed to provide a protective sleeve which comprises a one-piece front part, without any welding points. In addition, it is proposed to provide the protective sleeve with a curvature on its front side facing the sample medium.
WO 2010/063682 A1 discloses a temperature sensor having a thermocouple, which includes a sheathed fireproof cable including a sensor element attached to the cable end facing the sample medium. Electric connecting leads run through a metal tube of the sheathed cable for connecting the sensor element to an electronic evaluation unit. The disclosed temperature sensor is to be usable for temperatures up to 1200° C., and capable of sensing fast temperature changes. To this end, the sensor element consists of a thermo wire bead which protrudes from the sheathed cable and is received by a protective envelope that is attached to the end of the sheathed cable facing the sample medium. The protective envelope comprises a one-piece front part, without any welding points, and the sheathed cable is a flexible thin-walled metal tube with a small outer diameter, with the connecting leads running through the section thereof pointing away from the sample medium and creating the desired interface with an on-board electronic system. The attachment of the temperature sensor to the measuring point is realized by a special ring collar and a union nut.
A high-temperature sensor having a sensor element mounted in a protective tube is disclosed in EP 2 196 787 A2. To allow reliable measurements also in high-temperature environments, e.g. the exhaust gas system of a motor vehicle, the protective tube is surrounded by a reinforcement tube, the reinforcement tube is composed of material whose coefficient of thermal expansion is higher than that of the material from which the protective tube is formed. The reinforcement tube is fixedly connected to the protective tube in a first region of the protective tube, and an abutment element is also fixedly connected to the protective tube in a second region of the protective tube. The reinforcement tube, owing to its greater thermal expansion, comes into mechanical contact with the abutment element above a predefined temperature, whereby the high-temperature sensor is mechanically stabilized above this temperature. The space between the sensor element and the protective tube cap of EP 2 196 787 A2 is filled with a material having good heat-conducting properties. In this case, fine silicon powder may be used. The stabilizing mechanical contacting of the protective tube with the abutment element requires a minimum temperature, so that particularly directly in the starting phase, respectively, the non-high-performance operation the overall arrangement tends to vibrate which may put the reliability of the measuring arrangement at risk. The high-temperature sensor can be fixed in the exhaust gas system by means of a mounting pod.